New Satellites of Saturn


On July 26, 1995, Amanda Bosh of Lowell Observatory announced the discovery of at least two and perhaps four new satellites of Saturn. The discovery was based upon images taken by the Hubble Space Telescope on May 22, 1995, when Saturn's rings were tilted edge-on to the Earth. Every 15 years, Earth passes through Saturn's ring plane, an event that provides astronomers an opportunity to discover new satellites that are normally lost in the glare of the planet's bright ring system. Astronomers discovered 13 of Saturn's moons during ring-plane crossings from 1655 to 1980. Other satellites were identified during the Voyager spacecraft flybys of Saturn in the early 1980s.

Until the International Astronomical Union (IAU) gives names to these satellites their provisional designations and distance from Saturn's center are:

        Designation    (km)
        -----------  --------
        S/1995 S 1    137450
        S/1995 S 2    139700
        S/1995 S 3    141050
        S/1995 S 4    146450
The last two of these satellites are considered to be new satellites, but the first two could be existing known satellites. The uncertainty is due to inaccurate ephermerides of Pan, Atlas and Prometheus. Satellite S/1995 S 1 could be Pan, and S/1995 2 could either be Atlas or Prometheus.

The two satellites S/1995 S1 and S2 lie inside Saturn's thin, eccentric "F" ring. The third satellite S/1995 S3 lies just outside the F ring. It is apparently a shepherd satellite for the F ring which may account for the ring's braided appearance. The fourth satellite S/1995 S4 is 6,000 kilometers (3,700 miles) beyond the F ring. None of these satellites are larger than about 70 kilometers (43 miles) across.

Additional Hubble observations of Saturn, taken when the Earth crossed the ring plane on August 10, will provide more images that can be used to determine whether two of the four satellites detected by Hubble are truly new or not. If all four satellites are new, then the total number of known satellites orbiting Saturn will grow from 18 to 22.


Views of S/1995 S 3

S/1995 S 3 (GIF, 181K; caption)
This four-picture sequence (spanning 30 minutes) shows one of four new satellites discovered by the Hubble Space Telescope, in images taken of Saturn on May 22, 1995. This satellite is identified as S/1995 S3 and appears as an elongated white spot near the center of each image. The new satellite lies just outside Saturn's outermost "F" ring and is no bigger than about 15 miles across. The brighter object to the left is the moon Epimetheus, which was discovered during the ring-plane crossing of 1966. Both moons change position from frame to frame because they are orbiting the planet.

Saturn appears as a bright white disk at far right, and the edge-on rings extend diagonally to the upper left. To the left of the vertical line, each image has been processed to remove residual light from the rings and accentuate any faint satellites orbiting near the rings. The long observing times necessary to detect the faint satellites have resulted in Saturn's bright, overexposed appearance.

Credits: Amanda S. Bosh (Lowell Observatory), Andrew S. Rivkin (Lowell Observatory and University of Arizona/Lunar Planetary Lab), High Speed Photometer Instrument Definition Team (R.C. Bless, PI), and NASA.




Author: Calvin J. Hamilton.